1. Field of the Invention
The present invention relates to the construction and operation of packed columns used in separation processes such as absorption, stripping and distillation.
2. Description of Related Art
Packed columns are utilized as unit operations for performing separation processes such as absorption, stripping and distillation where there is a need to achieve mass transfer between gas and liquid phases. For example, in the absorption process, the packed column typically is used to remove a component from a gas stream by contact with a liquid sorbent.
In the oil and gas, and chemical processing industries, the separation process is often carried out to meet a predetermined product specification. One widely used application of the packed column is the removal of water from a gaseous hydrocarbon stream in conjunction with a gas-oil separation plant, or GOSP. During the production life of a well, the mix of crude oil and hydrocarbon gases produced to the surface and entering the transmission pipeline from the well head contains a variable proportion of water which can be in liquid and vapor form. It is important that the gas stream leaving the GOSP contains no more than a predetermined amount of water in order to avoid the corrosion of downstream piping, fittings, instrumentation and the like. The concentration of water is measured in pounds per million standard cubic feet (MMSCF).
As used herein, the terms “water load specification” or “specification” means the predetermined weight of water, e.g., pounds/MMSCF, of the treated wet gas leaving the wet gas separation unit operation.
The design of a water separation unit operation for a GOSP can be complex. A typical separator is provided with a stationary packing matter over which a liquid absorbent passes counter-currently with the upwardly moving wet gas from which water is removed. One commonly used sorbent is triethylene glycol (TEG), which will be referenced in the description that follows. Other absorbents include ethylene glycol, methyl ethylene glycol and methyldiethanolamine
As will be understood by one of ordinary skill in the art, the operating capacity and parameters of the numerous components employed in processing the feed streams must be taken into account and all are part of the initial design specification. As will also be apparent, any significant upstream changes will effect the downstream operations.
A typical separation unit operation of the prior art is illustrated in its essential simplified form in the schematic diagram of FIG. 1, the operation of which can be described as follows:                a. the high pressure wet gas (210) enters the inlet of the gas-liquid knock-out separator (100) for partial water and condensate removal (212);        b. the gas from the gas-liquid knock-out separator (214) enters the TEG contactor separation column (110) from the bottom of the column while the regenerated 99% pure lean TEG (256) enters from the top of the counter-current flow absorption process;        c. the dried treated gas (215) flows from the top of the packed contractor separation column (110) through a shell and tube exchanger (132) to further cool the lean TEG stream entering the contactor (110);        d. the rich TEG (216) is transferred to the regeneration system, where it is heated through the still condenser (120) before entering, via line 220, the flash drum (140) that is set at, e.g., 55 psig for flashing the hydrocarbons;        e. still condenser (120) also produces a vapor stream (218) that is removed from the system;        f. flash drum (140) produces a rich TEG stream (224) and a side stream (222);        g. the rich TEG (224) passes through multiple stages of filtration in the filter system (142A, 142B), the filter TEG stream (226) passes through purifier (144) and the purified stream (228) can pass to optional charcoal filter (146) and discharge stream (232) or as by-pass stream (230), after which the essentially pure stream (234) is heated by the lean TEG by passage through the rich/lean TEG heat exchanger (150);        h. the heated rich TEG (236) enters the TEG still (122) and into the reboiler (124) that is set at 392° F. from the TEG still for water removal;        i. a side stream of stripping gas (260) is fed to the reboiler (124) through the stripping column (125) to enhance the purity of the TEG stream (250) by lowering the partial pressure of the water;        j. the hot regenerated lean TEG (250) is collected in the accumulator vessel (126) and the recycle stream (252) passes through booster pump (128) to the rich/lean TEG heat exchanger (150); and        k. the cooled lean TEG stream (254) is pumped by the circulation pump (130) and the stream (255) is further cooled by the dry gas (215) passed through the heat exchanger (132) before the cooled stream (256) enters the contactor (110) at a pressure of about 393 psig. The warmer dry gas (216) is recovered from the separator unit for continued transmission.        
As is well known in the art, the capability of a packed column to achieve the desired predetermined specification for the treated gas stream is based upon the design parameters of the unit, including specifically its volumetric capacity, as well as the type of absorbent liquid used and the packing materials placed in the column. The configurations of the packing materials are selected to assure maximum contact and exchange between the liquid-bearing gas and the liquid sorbent. The design parameters for separation units are established and well-known in the art and the characteristics of the packing materials are published by their manufacturers to facilitate the design phase.
Common applications of packed columns in the oil and gas industry include contacting wet natural gas with triethylene glycol (TEG) liquid to dehydrate, or partially dry the gas to meet a required specification, and the sweetening of natural gas by removing sulfur compounds using methyldiethanolamine (MDEA) in counter-current flow over the packing material in the column.
A packed column is typically a cylindrical, vertically oriented vessel that includes a packed section that is filled with a specified packing material, of which there are many types. The packing can be placed randomly, as with Raschig rings, or the section can be filled with a specially designed structured packing material. The goal is to achieve the most effective contact between the two phases at a prescribed flow rate in order to achieve an efficient separation, while at the same time minimizing the size of the column and the corresponding volume of packing materials and sorbent in order to minimize the capital costs of construction and the continuing operational costs. As noted above, the size of the packed column, e.g., its diameter and height, are determined in view of the flow rate of the gas stream being treated, the initial load of the material that is to be removed by contact with the sorbent and the specification or maximum acceptable loading of the treated gas stream.
The features of a typical packed column (10) of the prior art will be described in more detail, with reference to FIG. 2. The pressure vessel includes upper portion (12), central portion (14) and lower end portion (16). The packed section (20) is filled with a packing material or materials which have been predetermined to provide the optimum contact conditions between the sorbent liquid and the material to be removed from the gas stream. The fresh or lean liquid sorbent (31) is maintained in a storage vessel (not shown) and is introduced via a liquid feed inlet (30) to the liquid distributor (32), which can include, as shown, a manifold with a series of disbursing nozzles (34) or, alternatively, a perforated tray (not shown) that distributes the sorbent liquid over the cross-sectional area of the packed section (20). Following its downflow over the packing material (22) and absorption of the compound(s) to be removed from the gas stream, the rich sorbent liquid (41) is retained by hold-up tray (40) and passes through a liquid outlet (42) to a sorbent regeneration unit (not shown), after which lean sorbent is returned to the sorbent storage vessel.
The gas stream (51) to be treated enters the column (10) via gas inlet (50) and is distributed through the lower cross-sectional area by gas inlet distribution device (52), which can be in the form of a manifold having a plurality of perforations (54) through which the gas (51) is emitted to provide a uniform upward flow. Any liquid present in the incoming gas stream is collected as condensate (58) in the bottom portion (16) of the column and can be discharged from the column (10) via valved condensate outlet (18). The bottom portion (16) of the column serves as an integrated gas-liquid knock-out separator that partially removes water and liquid condensates from the inlet gas stream.
The gas entering inlet (50) passes through gas chimney (44) which channels the gas stream directly into the packed section of the column without permitting it to travel through the accumulated liquid in the hold-up tray (40).
After passing in intimate contact with the sorbent liquid in the packed section (20), the treated gas accumulates in the upper gas disengagement section (36) in the top portion (12) of the column (10). The treated gas stream (37) is discharged through the gas outlet (38). The gas disengagement phase enables any liquid carried out in the gas stream to separate and return to be mixed with the incoming sorbent liquid feed (31).
A significant variable that must be taken into account in the design of the packed column is the anticipated load of water that is being carried in the gas stream to be treated. For example, as noted above, in the earliest stages of production from a new well, the water load may be relatively low. As hydrocarbon production continues, the well may have to be subjected to water injection in order to promote the flow from the surrounding reservoir rock into the wellbore for production to the surface. As a result, as the well matures, the amount of water produced and therefore present in the gas phase may increase significantly and at some point exceed the capacity of the packed column to reduce the water load and achieve the desired specification in the treated gas stream.
As will be understood by those of ordinary skill in the art, it is often not possible to reduce the flow of the gas stream into the packed column because of the continuous nature of the production and/or processing operations up and downstream from the packed column. The present invention addresses the problem of processing a gas stream in a separation unit where the load of the compound to be absorbed is variable and it is desired to maximize both the operating efficiency and the economics of the unit operation.
A further problem addressed is the reconfiguration or retrofitting of an existing separator unit operation that can no longer meet treated feed specifications due to changes in the incoming feed without incurring the major capital expenses associated with installing an entirely new separator unit in an existing larger complex unit operation.